1. Field of the Invention
The present invention relates to a wireless communication system for mutual communication among a plurality of wireless stations such as a wireless LAN (Local Area Network), a wireless communication apparatus, a wireless communication method and a computer program, and more particularly to a wireless communication system, a wireless communication apparatus, a wireless communication method and a computer program, in which a wireless network is configured by Ad-hoc communication without relationship between a controlling station and a controlled station.
2. Description of Related Art
More in detail, the present invention relates to a wireless communication system, a wireless communication apparatus, a wireless communication method and a computer program, in which a self-organized distribution type wireless network is formed in a communication environment preparing a plurality of channels, without interference between neighboring wireless systems and without relationship between a controlling station and a controlled station, and more particularly to a wireless communication system, a wireless communication apparatus, a wireless communication method and a computer program, in which interference from adjacent wireless systems at spatial positions is reduced by switching a channel by effectively using a plurality of frequency channels.
By configuring a LAN by interconnecting a plurality of computers, information such as files and data and peripheral machines such as a printer can be shared, and information exchange such as transfer of e-mails and data content can be made.
Conventionally, a wired LAN connection is generally realized by optical fibers, coaxial cables or twist pair cables. However, in this case, line laying works are necessary, it is difficult to configure a network with ease, and a cable distribution becomes complicated. The motion range of apparatuses after the LAN configuration is limited by cable lengths, which is inconvenient.
A wireless LAN has drawn attention as a system releasing a user from LAN wiring of a wired system. According to a wireless LAN, most of wired cables can be omitted in a work space such as an office so that communication terminals such as personal computers can be moved relatively easily.
In recent years, demands for a wireless LAN system are increasing considerably because of its high speed and low cost. Introduction of a personal area network (PAN) has been studied recently in order to perform information communication by configuring a small scale wireless network among a plurality of electronic machines existing about each person. Different communication systems have been stipulated by utilizing frequency bands unnecessary for licenses by supervisory offices, such as a 2.4 GHz band and a 5 GHz band.
As one of the standard specifications of wireless networks, IEEE (The Institute of Electrical and Electronics Engineers) 802.11 (e.g., refer to Non-patent Document 1), HiperLAN/2 (e.g., refer to Non-patent Document 2 or Non-patent Document 3), IEEE 802.15.3, Bluetooth communication and the like can be enumerated. The IEEE 802.11 standard has various wireless communication schemes such as the IEEE 802.11a standard and the IEEE 802.11b standard depending upon a difference of a wireless communication scheme and a frequency band in use.
In order to configure a local area network by using wireless technologies, a method is generally used by which one apparatus to be used as a control station called an “access point” or a “coordinator” is installed in an area and a network is formed under the collective control by the control station.
When information is transmitted from some communication apparatus in a wireless network having distributed access points, an access control method based on band reservation has been adopted widely by which a band necessary for transmitting the information is first reserved at an access point to use a transmission path without collision of information transmission with other communication apparatuses. Namely, synchronous wireless communication is performed by mutually synchronizing with communication apparatuses in the wireless network by distributing access points.
In a case where asynchronous communication is to be performed between communication apparatuses on the transmission side and reception side in a wireless communication system having access points, this wireless communication requires by all means wireless communication via an access point so that there arises the problem that a transmission path use efficiency is decreased.
As another method of configuring a wireless network, “Ad-hoc communication” has been devised in which terminals perform wireless communication directly and asynchronously. It can be considered that the ad hoc communication in which arbitrary terminals perform wireless communication directly without using a particular access point is suitable particularly for a small scale wireless network configured by a relatively small number of clients positioned near each other.
In a work environment in which information machines such as personal computers (PC) are prevailing and a number of apparatuses are mixedly used in an office, it can be supposed that a plurality of networks are configured in a superposed manner with scattered communication stations. In this state, if the wireless network uses a single channel, there is no room of recovering the situations that another system intrudes during communication and that the communication quality is degraded by interference or the like.
To avoid this, a conventional wireless network system generally adopts a method by which a plurality of frequency channels are prepared for coexistence of other networks and a communication operation starts by making a wireless communication apparatus serving as an access point select one frequency channel.
The multi-channel communication scheme of this type can maintain a network operation and realize coexistence of other networks by switching a frequency channel to be used, when another system intrudes during communication or a communication quality is degraded by interference or the like.
For example, a high speed PAN system of IEEE 802.15.3 also prepares a plurality of frequency channels usable by the system and adopts the algorithm that after a power is turned on, a wireless communication device selects a usable frequency channel by executing a scan operation for all usable channels in order to confirm whether or not there are devices which are transmitting a beacon signal as the Piconet Coordinator (PNC) around the wireless communication device.
In an ad hoc network of a self-organized distribution type without relationship between a controlling station and a controlled station, resource management of frequency channels is important in order to suppress as much as possible interference with nearby different wireless networks under operation. However, in order to change frequency channels used in the network at a time, a representative station called a coordinator or an access point is required to instruct a use channel to each terminal station. In other words, it is difficult to switch a frequency channel in the ad hoc network.
In HiperLAN/2 for example, a method of changing frequency channels at a time can be considered in order to selectively use a plurality of channels. As shown in FIG. 17, an AP (base station) as a central control station repetitively notifies a frequency channel change, and, at some timing, AP and an MT (mobile station) connected to AP switch the channels at a time. A judgment whether the channel is switched or not is determined initiatively by AP. Information to be used for the judgment is collected by following a process procedure illustrated in FIG. 18. Specifically, the process includes the following procedures: (1) upon an instruction from AP, the connected MT temporarily suspends communication, scans other frequency channels to evaluate the channel quality, and sends the results to AP; (2) upon an instruction from AP, AP temporarily stops the transmission on a broadcast channel, and the connected MT scans the frequency channel in present use, evaluates the channel quality and reports the results to AP; and other processes.
Bluetooth communication adopts a method by which a central control station called a master serving as a standard performs random frequency hopping to utilize squarely each frequency channel. Existence of the central control station called the master is essential for the network configuration and the central control station is used as the standard of a frequency channel hopping pattern and synchronization of a time axis direction. If the master extinguishes, the network formed until then is once disconnected so that a process of selecting a new master is necessary.
Also in a wireless LAN system of the IEEE 802.11 series, since a network is formed by using the frequency channel initially set by an access point, it is difficult to configure an ad hock network without disposing a base station. When communication with MT covered by AP operating at another frequency channel is to be performed, it is necessary to connect APs by wired LAN cables. Namely, if covering APs are not connected, communication is not possible even if wireless communication apparatuses (terminals) physically existing adjacent to each other are covered by different APs.
Also in a high speed wireless PAN system of IEEE 802.15.3, although it is possible to initially scan all frequency channels and search a neighbor coordinator, if an operation starts once at a particular frequency channel, it is not possible to grasp the use state of other frequency channels. Therefore, even if a neighbor Piconet using a different frequency channel exists, communication with a wireless communication connected to the Piconet is impossible.
As above, the conventional methods require a complicated mechanism such as timings of frequency channel switching, a setup process to be realized by message exchange for starting a frequency channel switching operation through mutual synchronization of participating terminals, and an adjustment process to be used for determining frequency channel switching. It is also essential that a central control station initiatively performing control exists, such as AP in IEEE 802.11 and HiperLAN/2 and a master in Bluetooth communication. If the central control station such as AP and a master extinguishes, some protocol process of selecting a substitute central control station or a manual setting change work is necessary, resulting in the problem that communication is intercepted during this process.
A wireless communication system has been proposed which determines a frequency channel by measuring not only interference of own channel but also interference of adjacent channels by using these channels (e.g., refer to Patent Document 1), this system realizing a multi-channel with involvement of a base station.
[Patent Document 1]
Japanese Patent Application Publication Hei 6-37762
[Non-Patent Document 1]
International Standard ISO/IEC 8802-11:1999(E) ANSI/IEEE Std 802.11, 1999 Edition, Part II: Wireless LAN Medium Access Control (MAC) and Physical Layer (PHY) Specifications
[Non-Patent Document 2]
ETSI Standard ETSI TS 101-761-1 V1.3.1 Broadband Radio Access Networks (BRAN); HIPERLAN Type 2; Data Link Control (DLC) Layer; Part 1: Basic Data Transport Functions
[Non-Patent Document 3]
ETSI Standard ETSI TS 101-761-2 V1.3.1 Broadband Radio Access Network (BRAN); HIPERLAN Type 2; Data Link Control (DLC) Layer; Part 2: Radio Link Control (RLC) sublayer
An object of the present invention is to provide an excellent wireless communication system, wireless communication apparatus, wireless communication method and computer program, which can properly configure a wireless network through ad hoc communication without relationship between a controlling station and a controlled station.
Another object of the present invention is to provide an excellent wireless communication system, wireless communication apparatus, wireless communication method and computer program, which can properly configure a suitable ad hoc network without any interference between communication stations in a communication environment preparing a plurality of channels.
Still another object of the present invention is to provide an excellent wireless communication system, wireless communication apparatus, wireless communication method and computer program, which can perform a channel access by effectively utilizing a plurality of frequency channels in a wireless network of a self-organized distribution type without relationship between a controlling station and a controlled station.
The present invention has been made in consideration of the above-described issues, and a first aspect of the present invention provides a wireless communication system for forming a network by a plurality of wireless communication apparatuses without relationship between a controlling station and a controlled station in a communication environment having a plurality of channels, characterized in that each wireless communication apparatus notifies beacon information containing seed information which is the basis of determining a beacon transmission timing and a transmission channel of its own station, when the wireless communication apparatus newly participates in the network, it identifies a channel switching pattern for beacon transmission on each channel, in accordance with the seed information in the information of a beacon received from another station, selects one candidate of own station from candidates not in use of the beacon transmission timing and transmission channel, and transmits the beacon information.
The “system” used in this specification means a logical collection of a plurality of apparatuses (or functional modules realizing specific functions) and does not specifically refer to whether each apparatus or function module is accommodated in a single housing.
In the self-organized distribution type wireless communication system, each communication station notifies beacon information in the transmission frame period and executes a scan operation for beacon signals from other stations to recognize the network configuration. In a self-organized distribution type network using multi channels, the transmission frames are multiplexed on the frequency axis as many as the number of usable channels. Therefore, a communication station cannot receive a beacon unless it moves to the same channel as that of another communication, at the beacon transmission timing, resulting in a problem that a new entry station is difficult to determine its own beacon transmission timing and transmission channel.
To solve this, the present invention configures a self-organized distribution wireless network using multi channels, by mutually notifying a channel transition pattern of beacon transmission to each communication station and by selecting the beacon transmission timing and transmission channel not subjected to interference.
In the multi-channel self-organized distribution wireless network of the present invention, each wireless communication apparatus determines the transmission channel for beacon information from those channels good to own station. If a communication station does not already exist in the neighboring area, an arbitrary channel switching pattern for defining own beacon transmission timing and transmission channel can be determined.
Further, at the time of a wireless communication apparatus trying to newly enter the network, the wireless communication apparatus determines the beacon transmission timing and transmission channel from the candidates excluding those having a bad communication quality relative to own station.
Still further, at the time of a wireless communication apparatus trying to newly enter the network, the wireless communication apparatus may determine the beacon transmission timing so as to make beacon transmission timings distribute time sequentially in the transmission frame on the selected channel. In order to effectively use communication resources, the transmission channel may be determined so as to distribute transmission channels on the frequency axis.
Further, in a case where the communication quality of some frequency channel is degraded in a steady state, a wireless communication station may determine the beacon transmission timing and transmission channel from the candidates excluding the degraded channels to switch the beacon transmission position and transmission channel.
A second aspect of the present invention provides a computer program written in a computer readable format for executing a process in a wireless communication environment without relationship between a controlling station and a controlled station, characterized by comprising a beacon transmission position control step of determining a beacon transmission timing and a transmission channel of own station, a seed information generating step of generating seed information uniquely corresponding to a channel switching pattern defining the determined beacon transmission timing and transmission channel of own station; a beacon information notifying step of notifying beacon information containing the generated seed information every predetermined frame period in own station communication range, and a beacon analyzing step of analyzing beacon information received from another station. In the computer program, in the beacon transmission position control step, at a time of new entry of a wireless communication apparatus, the channel switching pattern identified in accordance with the seed information extracted from the received beacon information is referred to and a candidate to be used by own station is selected from candidates of the beacon transmission timing and transmission channel not in use.
The computer program according to the second aspect of the present invention defines a computer program written in a computer readable format so as to realize a predetermined process on the computer system. In other words, as the computer program according to the second aspect of the present invention is installed in the computer system, a cooperative process is presented on the computer system to operate it as a wireless communication apparatus. A plurality of wireless communication apparatuses are activated to configure a wireless network so that similar operations and effects to those of the wireless communication system according the first aspect of the present invention can be obtained.
According to the present invention, an excellent wireless communication system, wireless communication apparatus, wireless communication method and computer program can be provided which can configure reliably a proper ad hoc network without any interference between communication stations in a communication environment preparing a plurality of channels.
Further, according to the present invention, an excellent wireless communication system, wireless communication apparatus, wireless communication method and computer program can be provided which can perform a channel access by effectively using a plurality of frequency channels in a self-organized distribution wireless network without relationship between a controlling station and a controlled station.
According to the present invention, interference from nearby wireless systems (e.g., systems in conformity with IEEE 802.11a and systems in conformity with HiperLAN/2 and HiSWANa) can be reduced effectively by switching the channel.
Further, according to the present invention, it is not necessary to use a complicated protocol for switching frequency channels at a time for terminals participating in a network. The present invention is also applicable when a plurality of frequency channels are required to be used effectively in a communication system providing a slot allowing each station to transmit data with priority after it transmits a beacon.
Other objects, features and advantages of the present invention will become apparent from the preferred embodiments of the present invention to be described later and the detailed description given in conjunction with the accompanying drawings.